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You need to go to a step smaller main jet in the carbs. Jetting is something that needs to be changed for elevation and temperature. The denser the air is, the more fuel you have to have to remain in the safe air/fuel ratio. So if you go up in elevation, there will be less air, so you need smaller jets. Also, if the temps go up, the air is not as dense; again, smaller jets.

The reverse is also true; go down in elevation, put in bigger jets, ride in colder weather, put in bigger jets. Manufacturers make jetting charts to keep you in the zone (and to keep the engine safe!) Going too small in the main jet will cause a lean condition that can lead to excessive heat and holes in the piston(s).

Start by locating the appropriate jetting chart, which should be in the owner's manual or available online or through a dealer. What year is the ZR? I have a manual for a 94 carbed, I could scan the chart if it's around that year.
 
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